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Analyzes 'isset(...)' function usages.<br />
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This inspection suggests using 'null !== ...', 'array_key_exists(..., ...)' instead.
In cases when isset is applied to array-accessing expressions, this inspection checks if concatenation is 
used to generate an index.<br />
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It should be noted that array_key_exists returns true even if array item is null; this inspection
is focused on verifying data structures' sanity and may be safely ignored in some cases.
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